Active crowbar for a switching magnet

ABSTRACT

A first electrical source is connected to an electro-magnet by silicon-controlled rectifiers (SCR&#39;&#39;s) to produce a rapid rise in current in the load. A second electrical source is then connected by SCR&#39;&#39;s to the load and the first is disconnected to produce a constant current in the load. A connection is then established to discharge the stored energy from the electromagnet, producing a rapid drop in current, and the second source is disconnected. The stored energy may be returned to the first electrical source.

United States Patent 1 1 Praeg Dec. 4, 1973 1 ACTIVE CROWBAR FOR A SWITCHING MAGNET [75] Inventor: Walter F. Praeg, Palos Park, Ill.

21 Appl. No.: 269,004

3,127,573 3/1964 Weil 307/107 3,267,299 8/1966 Bartelink 307/109 3,133,227 5/1964 Brown et a1. 3l5/5.42

Primary ExaminerRobert K. Schaefer Assistant ExaminerM. Ginsburg Attorney-John A. Horan [5 7 ABSTRACT A first electrical source is connected to an electromagnet by silicon-controlled rectifiers (SCRs) to produce a rapid rise in current in the load. A second electrical source is then connected by SCRs to the load and the first is disconnected to produce a constant current in the load. A connection is then established to discharge the stored energy from the electromagnet, producing a rapid drop in current, and the second source is disconnected. The stored energy may be returned to the first electrical source.

8 Claims, 5 Drawing Figures (O/VTROL ME/I/VS [52] US. Cl. 307/108 [51] Int. Cl. H03k 3/00 [58] Field of Search 307/106, 107, 108, 307/109, 110

[56] References Cited UNITED STATES PATENTS 3,144,567 8/1964 Moehlmann 307/109 X S W/TC/fl/VG MEHIVS SOURCE sw/rc/fl/va NEH/V5 sou/me PATENTED DEE 41975 SHEEY 1 [IF 3 6 L w 5 MM W f f 5 H W Mm L 6 6 E! C k WMB 5% S W 5 2/ l /3 y M I b W Z H 1 1% 1 m 2 3 3 y H w n a 3 w E x 5 2 R a k A 2 Z 0 7 2 3w PATENTEUUEC 4 I973 SHEET 3 0F 3 T/ME l/v MILL/SECONDS 1 ACTIVE CROWBAR FOR A SWITCHING MAGNET CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to a method and means for control of pulses of electric current in an inductive load. In particular, it relates to a method and means of generating in an electromagnet pulses of current having a rapid rise, a constant-value flat top within a high degree of precision for a predeterminedperiod of time, then dropping rapidly to zero.

Control of pulsed magnetic fields by the generation of current pulses is required for proper operation of switching magnets in particle accelerators. The basic requirement for generating current pulses is an energy source that can store the necessary energy and can transform and deliver it to a load in a relatively short time. This is also a requirement in delivering energy to radar transmitters, and a picturesque term that was applied to radar pulse modulators during World War II is still used to describe a process for such rapid delivery. The term is crowbar and is used to describe an apparatus that suddenly applies a short circuit in a network, in analogy to the dropping of a crowbar across a pair of electrical terminals. This effect has been achieved in the past by the use of gas tubes such as thyratrons-and ignitrons and also of solid-state devices such as siliconcontrolled rectifiers. Typical operation comprises connecting a charged capacitor or bank of capacitors through a switch in series with a low-resistance inductive load such as a magnet. The design parameters are chosen to approximate an oscillatory RLC network, and the time necessary to achieve the first peak of current, the quarter period, is one parameter in the design. When current is near the first peak, the capacitor bank is crowbarred by the application of a short circuit, the crowbar, across both the capacitor bank and the inductive load. This fixes the voltage across the inductive load at the value of the iR drop in the crowbar. Since the voltage across the inductive load is also measured by L di/dt, where L is the series inductance and di/dt is the time rate of change of the current through the load, it is evident that for a fixed inductance L, a short circuit results in a low di/dt and thus a relatively constant current. The state of the art is summarized in the attached table which is abstracted from Table'6.II of Pulsed High Magnetic Fields by Heinz Knoepfel, North- Holland Publishing Company, Amsterdam-London, and American Elsevier Publishing Company, Inc., New York (1970).

(projected) NB. All banks can be crowbarred at current maximum with L/R decay times of the order of microsec. (with the exception of the Livermore bank).

The meaning of the footnote to the table is that application of a crowbar at the current peak initiates an exponential decay with the specified time constant, resulting in a current variation that is slow in comparison with the rate obtained as the current rose to its peak. The slope of the current waveform immediately after crowbarring is inversely proportional to the L/R decay time, which thus sets a limit upon the constancy with which current can be maintained by crowbarring.

For an application such as beam switching in a particle accelerator, conventional crowbar circuits have disadvantages associated with the exponential decay of current. Ideal operation of a switching magnet would comprise an instantaneous rise to a constant value and an instantaneous drop to zero. The finite rise and fall time of current pulses in actual magnets either results in'loss of beam during the buildup and decay of the pulse or else requires scheduling the buildup and decay while the beam is off. Variations about the desired constant value of current, and thus of magnetic field, cause dispersion of the beam into a spread deflection angle. To minimize this dispersion and beam loss, it is desirable to maximize the approach to the ideal pulse. Thus, the prior-art crowbar circuits cannot produce through an electromagnetic load a current pulse having a rapid rise time, an extremely flat top portion (within 0.005 percent), and a rapid drop-off to zero.

It is therefore an object of the present invention to provide a method and means of generating a current pulse having an extremely flat top in an inductive load.

It is a further object of the present invention to provide a current in an inductive load having a rapid rise, a flat top, and a rapid return to zero.

It is a further object of the present invention to provide a method and means of generating a current pulse in a switching magnet for a particle accelerator, said current pulse having a portion that remains within 10.005 percent for a period of 5 milliseconds.

It is a further object of the present invention to provide a method and means of returning electrical energy stored in an electromagnet to a source to terminate a current pulse.

SUMMARY OF THE INVENTION The present invention is a method and means of generating in an inductive load a current pulse having a rapid rise, a flat top, and a rapid drop. A first electrical source is connected to the inductive load to produce a rapid rise in current. At a predetermined value of current, a second electrical source is connected to the inductive load and the first source is disconnected from the load. After a predetermined time, a loaddischarging electrical connection is established and the second electrical source is disconnected from the load.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an apparatus for the practice of the present invention.

FIG. 2 is a circuit diagram showing a preferred apparatus for the practice of the present invention.

FIG. 3 is a plot of current and voltage waveforms in the circuit of FIG. 2.

FIG. 4 is a plot of current waveforms in certain elements of the circuit of FIG. 2.

FIG. 5 is a plot of current waveforms in a switching magnet.

DESCRIPTION OF A GENERAL EMBODIMENT FIG. 1 is a block diagram illustrating a general apparatus for the practice of the present invention. Load is an inductive device having low resistance, such as an electromagnet. A source 12 is connected electrically by switching means 13 to load 10 to begin the rise of electric current in load 10. At this time and during the buildup of current in load 10, switching means 15 and 16 are in an open-circuit condition. All of the switching means l3, l5 and 16 are controlled by control means 17.

When the electric current in load 10 reaches a desired value, usually just past a peak value, control means 17 operates switching means 15 to connect a second source 18 to load 10. Switching means 15 in combination with source 18 comprises an active crowbar which maintains the electric current in load 10 at a highly constant value. At the time of application of source 18 to load 10, control means 17 operates to open-circuit switching means 13 between source 12 and load 10, thereby electrically disconnecting load 10 from source 12.

When it is desired to end the current flow in load 10, control means 17 operates to close switching means 16 and to open switching means 15. Source 12 is thereby connected to load 10 in an electrical polarity opposite to that originally applied. This causes much of the stored energy in load 10 to be returned to source 12. The time sequence of operations is concluded when load 10 is discharged and switching means 16 opens. All connections are thus returned to their original condition with no current flow through any of the switching means or through load 10. The cycle of operation is thus as follows: (1') buildup of load current through switching means 13; (2) crowbarring of loadcurrent through switching means 15; and (3) decrease of load current through switching means 16.

DESCRIPTION OF PREFERRED EMBODIMENT A circuit diagram of a preferred apparatus using silicon-controlled rectifiers to accomplish switching is shown in FIG. 2. Load 10 is here indicated as an ironcore inductor, which is a typical circuit representation of an electromagnet. Load 10 is in series with current sensor 11, a precision shunt. High-voltage source 21 and paralleled capacitor 22 are connected by silicon-controlled-rectifier (SCR) pair 23 to load 10, in response to a pulse from pulse generator 14, to produce a fast-rising electric current in load 10. Pulse generator 14 is actuated by an external trigger signal. Crowbar pulse generator 19 actuates SCR pair 25 at a desired condition of current through load 10 as sensed by current sensor 11, which is a precision shunt. A preferred method of operation is to compare in crowbar pulse generator 19 the condition of the current as sensed by current sensor 11 with a reference signal, and to operate SCR pair 25 after the current in load 10 has passed a peak value, so that SCR pair 23 will disconnect automatically from source 21 because of the unidirectional nature of conductivity of the SCRs. This process can be described as automatic commutation of the current.

The result of operating SCR pair 25 is to connect low-voltage source 26 and, in parallel with it, capacitor 27 to load 10. In normal operation, the buildup of current in load 10 during the previous time interval through SCR pair 23 has reduced the voltage across capacitor 22 to a low value that is opposite in polarity to the polarity of its original charge. Connection of source 26 to load 10 thus applies a back bias to SCR pair 23, reducing their electric current to zero. Source 26 and capacitor 27 now supply all the current flowing through load 10, and the variation of that current can be determined by the parameters of source 26 and capacitor 27 independently of the parameters of source 21 and capacitor 22, which are disconnected from load 10.

After a desired time has elapsed, pulse generator 20 is caused to trigger SCR pair 29, in response to an external timing signal. Load 10 is thus connected to source 21 and capacitor 22 in a polarity that is opposite in sense to that first applied through SCR pair 23. SCR pair 25 will now be back-biased and will become opencircuited. Electromagnetic energy stored in the form of magnetic fields in load 10 is now returned to source 21 and capacitor 22, restoring much of the charge on capacitor 22. When the current in load 10 reaches zero, SCR pair 29 becomes an open circuit and the system is quiescent, with all SCR pairs not conducting and with no current flow in load 10. The results of energy dissipation during the pulse cycle are now apparent only in reductions in the stored energy in and thus the voltages across capacitors 22 and 27. High-voltage source 21 will recharge capacitor 22 and low-voltage source 26 will recharge capacitor 27 to restore the original conditions to the circuit.

It should be appreciated that circuit arrangements other than those of FIG. 2 could be used to practice the present invention according to the structure of FIG. 1. For example, any switching means l3, l5, and 16 could be used to make and break the desired connections, including, but not limited to, relays, thyratron tubes, and knife switches. Sources 12 and 18 could be any sources of electrical energy, including, but not limited to, batteries, capacitor banks, and generators.

Further understanding of this invention may be gained from noting FIG. 3, which is a plot of various current and voltage waveforms in the circuit of FIG. 2 as a function of time. Load current curve 31 is a plot of current i, the current in load l0, as a function of time. Current 1' is seen to be zero before time t At time t SCR pair 23 in FIG. 2 has been triggered by an external signal, and current i rises to a peak value and starts to decrease. Time 1, marks the end of the rise interval and the beginning of the flat top. At time t, SCR pair 25 is triggered responsive to the sensed current direction and amplitude, and current i is seen to stay essentially constant due to the crowbarring action heretofore described. At time SCR pair 29 is triggered responsive to an external signal, and current i enters its fall interval. Current i continues to fall until it reaches zero at time t;,, after which time it remains at zero.

Load voltage curve 32 is plotted on the same time axis as current i, and is taken with a reference at the top of load 10. Load voltage curve 32 is seen to be at a high value associated with the voltage across high-voltage source 21, at time t,,. After time t during the rise interval of the load current 1', load voltage drops through zero to a negative value at time t,. At time I, load voltage curve 32 jumps discontinuously. Between times t,

5. and load voltage curve 32 remains at an essentially constant value equal to the voltage across low-voltage source 26. At time t the load voltage begins areturn to the magnitude of its original value. Because of the reversal in directions associated with the connection of third SCR pair 29. load voltage curve 32 approaches a negative voltage as load 10 discharges its energy back into source 21 and capacitor 22. This can be seen'by referring to curve 33, which is the voltage across capacitor 22 with voltage reference at the top. During the rise, curve 33 coincides with load voltage curve 32. During flattop, which is the interval t,' t capacitor 22 is disconnected from the load and its voltage remains constant. During fall, capacitor 22 is connected across load 10 in the opposite sense, and curve 33 is thus the reflection of load voltage curve 32 about the time axis.

Further illustration of the function of circuit elements is provided by FIG. 4, which shows currents in various elements during a current pulse. Currentwaveform 4-1 is the current through'SCR pair 23. Current waveform 42 is the current through SCR pair 25. Current waveform 43 is the current through SCR pair 29. The sum of these three currents comprises load current curve 31 as shown in FIG. 3. The commutation of currents as the load is switched from one source to another is exaggerated at times t. and to show that the fast changes of current flow in opposite directions add to produce the smooth waveform of load current curve 31.

The principles of the present invention were'used in an actual switching magnet used to deflect a beam of protons at 12.5 GeV at the Zero Gradient Synchrotron at Argonne National Laboratory. This particular switching magnet was designedto produce its rated magnetic field at a current of 1800 Amperes. The magnet has a coil resistance of 0.020 Ohms and a d-c inductance of 0.053 H. It was desired to maintain a flat top of current within 0.005 percent, a tolerance of 10.090 A, for a period of milliseconds. The capacitor 22 of FIG. 2 was a bank totalling 1200 microfarads, charged to 12.9 kilovolts. The switching magnet comprising load and first capacitor 22 in this embodiment thus had a resonant frequency of approximately Hz, providing a quarter-cycle time of 12.5 ms. This is approximately the length of the rise'time from t to t. for the particular embodiment. At time t the desired voltage across second capacitor 27 was calculated to be 1800 A X 0.053 Ohms 95.4 volts, the amount needed to hold load current i constant at 1800 A. Several values of the capacitance of capacitor 27 and of the initial voltage across capacitor 27 were used, andthe results are shown in FIG. 5, which is a plot of load current versus time for several conditions. Parameters in FIG. 5 are the initial voltage on second capacitor 27 and the capacitance of second capacitor 27. The curves have the parameters listed in the following table.

Capacitance 2.25 F 4.50 F 9.00 F Initial voltage 95.400 V (l00%) 51 52 53 96.354 V 00%) 54 55 56 97.308 V (102%) 57 58 59 The values actually chosen for use were those providing curve 57, namely, 97.308 V and a capacitance of In the practice of this invention, various modificaple, economical use of silicon-controlled rectifiers may dictate the use of a plurality of such devices in series or in parallel or both in place of each indicated one of an SCR pair, according to requirements of voltage rating or current capacity. It may be desirable to insert'current-limiting inductances in series with some or all SCRs, and it may be advantageous to use capacitors or RC networks across SCRs to facilitate switching.

It will be understood that the invention is not to be limited by the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for supplying an inductive load with a pulse of current comprising: 7

l. a first electrical-energy source means operably connectable to said inductive load;

2 first switching means to effect an electrical connection of said first electrical-energy source means to said inductive load to produce a rapidly rising electric current to a predetermined value in said inductive load;

3. a second electrical-energy source means operably connectable to said inductive load;

4. second switching means to effect an electrical connection of said second electrical-energy source means to said inductive load and disconnect said first electrical-energy source means from said inductive loadto provide a constant current at said predetermined value in said load; and

5. means for reducing electric current in said inductive load to zero and disconnecting said second electrical-energy source means from said inductive load, a pulse of current being thereby produced in said inductive load having a rapid rise, a flat top, and a rapid drop.

2. Theapparatus of claim 1 wherein said first electrical-energy source means comprise a capacitor in parallel connection with an electricalvoltage source.

3. The, apparatus of claim 2 wherein said second electrical-energy source means comprise a capacitor in parallel connection with an electrical voltage source.

4. The apparatus of claim 1 wherein said first and second switching means comprise silicon-controlled rectifiers.

5..The apparatus of claim 1 wherein said means for reducingelectric current in saidinductive load to zero comprise a silicon-controlledrectifier connected to discharge said inductive load into said first electricalenergy source means.

6. An apparatus for controlling electric current in an electromagnet comprising:

1. a first source of electrical energy;

2. a first silicon-controlled rectifier connected electrically in series with said first source'and said electro-magnet;

3. first triggering means connected to said first silicon-controlled rectifier to render said siliconcontrolled rectifier electrically conductingto produce a rise in current in said electromagnet to a predetermined value;

4. a second source of electrical energy;

5. a second silicon-controlled rectifier connected electrically in series with said second source and said electro-magnet;

6. current-sensing means connected to sense electric current in said electromagnet;

7. second triggering means responsive to said current-sensing means to trigger said second siliconcontrolled rectifier in response to a signal generated from said current-sensing means to produce a constant current in said electromagnet;

8. a third silicon-controlled rectifier connected electrically to said electromagnet and to said first source of electrical energy in a polarity to effect discharge of stored electromagnetic energy in said electomagnet into said first source; and

9. third triggering means to render said third siliconcontrolled rectifier conducting to effect discharge of said stored electromagnetic energy into said first source. 7. A method of generating a pulse of current in an inductive load comprising the sequential steps of:

charging said second current comprises returning electrical energy associated with said second current to said first source of electrical energy. 

1. An apparatus for supplying an inductive load with a pulse of current comprising:
 1. a first electrical-energy source means operably connectable to said inductive load;
 2. first switching means to effect an electrical connection of said first electrical-energy source means to said inductive load to produce a rapidly rising electric current to a predetermined value in said inductive load;
 3. a second electrical-energy source means operably connectable to said inductive load;
 4. second switching means to effect an electrical connection of said second electrical-energy source means to said inductive load and disconnect said first electrical-energy source means from said inductive load to provide a constant current at said predetermined value in said load; and
 5. means for reducing electric current in said inductive load to zero and disconnecting said second electrical-energy source means from said inductive load, a pulse of current being thereby produced in said inductive load having a rapid rise, a flat top, and a rapid drop.
 2. first switching means to effect an electrical connection of said first electrical-energy source means to said inductive load to produce a rapidly rising electric current to a predetermined value in said inductive load;
 2. generating in said inductive load, from a second source of electrical energy, a second current remaining substantially constant at said predetermined value of said first current; and
 2. a first silicon-controlled rectifier connected electrically in series with said first source and said electro-magnet;
 2. The apparatus of claim 1 wherein said first electrical-energy source means comprise a capacitor in parallel connection with an electrical voltage source.
 3. first triggering means connected to said first silicon-controlled rectifier to render said silicon-controlled rectifier electrically conducting to produce a rise in current in said electromagnet to a predetermined value;
 3. discharging said second current to produce a rapid drop of said second current in said inductive load to a magnitude of zero.
 3. a second electrical-energy source means operably connectable to said inductive load;
 3. The apparatus of claim 2 wherein said second electrical-energy source means comprise a capacitor in parallel connection with an electrical voltage source.
 4. second switching means to effect an electrical connection of said second electrical-energy source means to said inductive load and disconnect said first electrical-energy source means from said inductive load to provide a constant current at said predetermined value in said load; and
 4. a second source of electrical energy;
 4. The apparatus of claim 1 wherein said first and second switching means comprise silicon-controlled rectifiers.
 5. The apparatus of claim 1 wherein said means for reducing electric current in said inductive load to zero comprise a silicon-controlled rectifier connected to discharge said inductive load into said first electrical-energy source means.
 5. means for reducing electric current in said inductive load to zero and disconnecting said second electrical-energy source means from said inductive load, a pulse of current being thereby produced in said inductive load having a rapid rise, a flat top, and a rapid drop.
 5. a second silicon-controlled rectifier connected electrically in series with said second source and said electro-magnet;
 6. current-sensing means connected to sense electric current in said electromagnet;
 6. An apparatus for controlling electric current in an electromagnet comprising:
 7. second triggering means responsive to said current-sensing means to trigger said second silicon-controlled rectifier in response to a signal generated from said current-sensing means to produce a constant current in said electromagnet;
 7. A method of generating a pulse of current in an inductive load comprising the sequential steps of:
 8. The method of claim 7 wherein said step of discharging said second current comprises returning electrical energy associated with said second current to said first source of electrical energy.
 8. a third silicon-controlled rectifier connected electrically to said electromagnet and to said first source of electrical energy in a polarity to effect discharge of stored electromagnetic energy in said electomagnet into said first source; and
 9. third triggering means to render said third silicon-controlled rectifier conducting to effect discharge of said stored electromagnetic energy into said first source. 